High vacuum all-metal waveguide valves have been in use extensively at Stanford Linear Accelerator Center ("SLAC") for more than 20 years. The original SLAC type valves used therein employed a vacuum seal and a resonant iris coupling the broad walls of parallel waveguides. The vacuum seal was achieved by forcing a circular plunger with a stepped knife-edge into an indium filled groove which was concentric with the circular coupling iris. By periodically remelting the indium, the valve seal was made reusable. The original valves worked adequately below a peak power level of about 30 MW.
There are employed in Beijing Electron Positron Collider ("BEPC") waveguide valves having a deeper seal groove and a hemispherical sealing surface instead of the knife-edge seal used in the original SLAC type valves. The vacuum seal in these valves often failed due to the indium adhering to the sealing surface, tearing the indium seal. Moreover, the sharp jagged points resulting from the indium adhesion caused radio frequency ("RF") breakdown during a high power operation.
Recently, at SLAC, a modified waveguide valve capable of overcoming the shortcomings of the original SLAC type waveguide valve has been developed. In the modified waveguide valve, the knife-edge indium seal is completely eliminated: instead an O-ring seal mechanism, which is transported to a RF-free environment during a high power operation, is employed. The continuity of wall currents on the contact surface is achieved through a shorted choke joint followed by a metal-to-metal RF seal. However, the modified waveguide valve still employs the resonant iris coupling the broad walls of the parallel waveguides and it works adequately below a peak power level of about 65 MW.